Condensing mode (CM) technique and Super condensing mode (SCM) technique raise productivity of gas phase process of ethylene polymerization by from 50 to 100 percent and from 60 to 300 percent, respectively. Activity of conventional catalysts for gas phase process of ethylene polymerization is typically in the range from 3,000 to 5,000 grams polyethylene per gram catalyst so that they are not suitable for these new techniques. Therefore, big companies in the world have been researching higher-activity catalysts useful for gas phase polymerization of ethylene.
Increasing atom ratio of magnesium to transition metal element and increasing amount of transition metal element in catalyst are effective methods for improving activity of catalyst. EP0771820 discloses a supported catalyst and improves conventional impregnating processes. First, an impregnating solution containing magnesium, titanium and an electron donor is formed, the impregnating solution is mixed with a carrier, and the solvent is evaporated, then the carrier is impregnated with the impregnating solution with same composition once more, and the solvent is evaporated. Impregnating may be repeated for many times as required. Repeated impregnating can insure magnesium chloride distributing in the pore of the carrier as uniformly as possible. As a result, a high ratio of Mg/Ti is obtained, and the resulting catalyst has less agglomerates and high activity. However, the catalyst preparation procedure needs repetitious impregnating (at least two times), and concentration and volume of the impregnating solution are necessary to be strictly controlled based on the varying of pore volume of carrier, which renders the preparation procedure quite complicated. Moreover, amount of the electron donor is greatly increased, therefore, the production cost is increased, and load of solvent recycling unit is also increased.
Another method for obtaining high ratio of Mg/Ti so as to prepare high-activity catalyst is spray-drying process. U.S. Pat. No. 4,293,673 and WO 01/05845 disclose methods for preparing catalyst by spray drying. Said spray drying processes are performed by forming uniform slurry from fumed silica as carrier and catalyst components, followed by spray drying. Since surface area of fog drops is large, drying time of catalyst is short, catalyst particles are dispersed uniformly, and the morphology of catalyst particles is easily controlled, thus spray drying is favorable for improving the activity of catalyst. Moreover, when same impregnating solution is used, one time higher Mg/Ti ratio than that obtained by conventional impregnating processes is obtained by spray drying, meanwhile, larger amount of titanium and no agglomerates are obtained. But the catalyst particles produced by spray drying process must be collected in a mineral oil, therefore, equipment cost for catalyst production is increased and production operation is more difficult. U.S. Pat. No. 6,303,716 discloses a process for preparing a high-activity solid catalyst for producing all-density polyethylene by gas phase polymerization. Said process comprises the step of: using powdered magnesium as raw material to form a magnesium halide in nascent state; reacting the magnesium halide in nascent state with titanium tetrachloride, diethyl aluminum chloride in the presence of tetrahydrofuran to form a complex; supporting the complex onto silica carrier; and heating and drying the silica carrier loading said complex to form a solid main catalyst component. The catalyst is advantageous for a simple preparing procedure and high catalytic activity. In practical production, the catalytic activity can be greatly increased by increasing the amount of titanium in the catalyst. However, with the remarkable increase of amount of titanium in the catalyst, amount of magnesium chloride is correspondingly increased, accordingly the viscosity of impregnating solution is larger so that overmuch magnesium chloride is deposited on the surface of the carrier, and the formation of agglomerates is serious. As a result, the yield of catalyst product is inevitably reduced and the production cost is increased in industrial production.